1. Field of the Invention
This invention relates to heating systems including furnaces having fuel-fired burners, and, more particularly, to a control arrangement which provides a safety interlock between a stack damper control apparatus and fuel supply apparatus for such systems.
2. Description of the Prior Art
Heating systems employing furnaces having fuel-fired burners require a vent stack to conduct combustion products away from the burner. Automatically controlled stack dampers are generally used in the ventilation stacks to permit the stacks to be closed when the furnace is not operating to minimize heat losses when the furnace is not operating. However, for safe operation, it is necessary that the stack damper be open in advance of each operation of the burner. Accordingly, systems in which automatic dampers are used generally include a control arrangement which provides an interlock between the damper control mechanism and fuel supply apparatus of the system to assure that the damper is fully open before the burner operates and is closed after the completion of the operation of the burner.
In one known arrangement in which a primary burner control is conditional on and subsequent to the opening of a stack damper, a drive motor is energized in response to a request for heat to drive the damper to an open position, and limit switches complete the burner circuit and deenergize the drive motor when the damper reaches the fully open position. The drive motor is reenergized at the end of the heat run to move the damper to the closed position, a further switch deenergizing the motor when the damper reaches the closed position. Movement of the damper from the fully open position permits a limit switch to interrupt the burner circuit. A time lag is provided between the interruption of the burner circuit and the closing of the damper to allow volatiles to be purged from the furnace following operation of the burner.
When operating properly, systems such as the type referred to above provide the desired interlock between the stack damper and the fuel supply apparatus. However, under certain failure conditions, such as the welding together of contacts of the limit switches, or, when cam-operated switches are used, the cams becoming loose and shifting out of place, the fuel supply valves may be energized while the vent stack is closed.
Therefore, it would be desirable to have a control arrangement for use in a heating system which provides a safety interlock between a stack damper control apparatus and fuel supply apparatus of the system which prevents operation of the fuel supply apparatus whenever the damper is in a position other than a fully open position, or in the event of an unsafe failure of the control apparatus.